Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
AND ASTROPHYSICS 1.4.1996 Your thesaurus codes are: 02.01.2; 02.12.1; 06.20.1; 06.21.1; 08.01.3; 08.12.1 Estimating the total radiative power output from the hot outer layers of late-type stellar atmospheres: How many lines are required? J.G. Doyle Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland email: [email protected] Received date, accepted date Abstract. The total radiative power output from the hot outer layers of six late-type stars (of dierent spectral types and luminosity class) is derived via an emission measure technique. This analysis was based on observational data from the spectral range 100 A to 3000 A obtained as a result of three separate satellite missions, i.e. Hubble Space Telescope, International Ultraviolet Explorer and the Extreme Ultraviolet Explorer. Solar observational data of active regions, coronal holes, sunspots, `quiescent' regions and ares were also used. Based on the derived total power output from all of these dierent plasma, it is shown that a linear relationship involving a single transition region line can be used to provide an accurate estimate of the total power output as previously shown for the Sun. The derived relationship does not include losses due to hydrogen, nor the UV continuum which can be a large contributor, particularly for very active stars. Key words: hot outer layers { magnetic activity { emission measure { radiative loss function 1. Introduction Modern ultraviolet/extreme ultraviolet spectroscopic observations, see for example the extreme ultraviolet data for Procyon given by Drake et al. (1995), provide an idea on both the complexity and number of spectral lines in the hot outer layers of late-type stellar atmospheres (by hot outer layers we refer to the upper chromosphere, the corona and the intermediate transition zone). These strong emission lines are generally considered to be the main contributors to the radiative cooling at those atmospheric heights. Studies based on single spectral lines has Send oprint requests to : J.G. Doyle however lead a number of authors to propose that magnetic activity levels saturate and yield the so-called `saturated atmospheres' (e.g. Vilhu 1987; Linsky 1991). This conclusion has been based mostly on chromospheric lines, e.g. Mg II h&k (Doyle 1987; Mathioudakis & Doyle 1992), Ca II H&K (Schrijver 1987) and H (Herbst & Miller 1989). There is however no convincing evidence that a saturation exists at X-ray wavelengths. Generally, for transition region lines and broad-band X-rays uxes, there is a poorly dened upper envelope separately for main sequence stars and other more active stars such as contact binaries (e.g. Vilhu 1987). Whether this saturation really corresponds to a saturation in surface magnetic elds is not clear and leads us to ask the question: how many lines are required in order to estimate the total radiative power output from the hot outer layers of late-type stars? You may be tempted to give values ranging from several hundred to thousands of lines. For example, taking data in the ultraviolet & extreme ultraviolet down to the X-ray region, will cover plasma temperatures ranging from 104 to 107K, i.e. from the chromosphere to the corona. Then, addition of these individual lines uxes gives the total radiative output. This would be a rather time-consuming method to employ and is currently impossible for targets other than the Sun, since even using IUE (International Ultraviolet Explorer), EUVE (Extreme Ultraviolet Explorer) & HST (Hubble Space Telescope), still leaves spectral regions unobserved. However, are there alternative methods? 2. Methods used & sources of observational data Ultraviolet data has been available for several years (as a result of IUE) for many late-type stars. However, it is only very recently that objects other than the Sun have been observed with sucient sensitivity to resolve spectral lines in the EUV region. The Extreme Ultraviolet Explorer has The derived emission measure curve for Cen (solid line) with that derived from the EUVE data alone from Schrijver et al. 1995 (dashed line), (b) the observed over predicted line ratio and (c) the total power output (in erg s?1 ) as a function of temperature Fig. 1. (a) An emission measure curve for the `average' quiet Sun, (b) the observed to predicted line ratio and (c) the resulting surface radiative power output (in erg cm?2 s?1 ) as a function of temperature Fig. 2. (a) now provided data for a range of late-type stars, including by Doyle & Keenan (1992). The emission measure is derived via the simple relation M dwarfs, RS CVn binaries and solar-type stars. In order to estimate the total radiative losses, we will use `the emission measure technique'. This is a quantity Ne2 dV = Pob () 1026:00 cm?3 (2) Ppr () (Ne2dV ) which is related to the line power via where Pob() is the observed power in ergs s?1 of a line Z N(H) (1) at wavelength and Ipr () is given by Doyle & Keenan P() = h Nj AjiNi A N Ne2 dV erg s?1 (1992). Note that the above denition of the emission e measure is slightly dierent from that used by Jordan et where Nj is the population of the upper level j normalized al. (1987), since we integrate in temperature intervals of to the total population of all levels in the ion, Aji the tran- log Te = 0:1 compared to log Te = 0:3 used by Jorsition probability from level j to level i, Ni the ionization dan et al. Using a sequence of suitable lines (based on fraction of the ion, A the element abundance relative to the availability of suitable observational data and on the hydrogen and N(H) the abundance of hydrogen relative accuracy of the existing atomic data) from species of difto the electron density. ferent excitation (i.e. temperature of formation ranging In order to aid in the construction of these emission from chromosphericRlines to coronal lines), a trial volume measure curves, Raymond & Doyle (1981a) presented a emission measure ( Ne2 dV ) versus temperature may be table of UV & EUV line emissivities, which were updated derived. We then integrated the intensities of all our lines prove the t. Further details on the technique is given in Raymond & Doyle (1981b), while the atomic data sources are given in Doyle & Keenan (1992). The biggest source of error in the derived EM curves is the abundances. Throughout, we used the coronal abundances as given by Meyer (1985), i.e. C, N, O & Ne dier by factors of 2 to 3 from the solar photospheric abundances. The coronal abundances given by Feldman et al. (1992) are dierent from those given here and are closer to the photospheric abundance of Meyer (1985). Here, we consider observational data from the above three missions (i.e. IUE, HST & EUVE) for six dierent late-type objects: AU Mic (an active dMe star), Cen and Ori (G2 & G0 solar-type stars), Capella and Gem (G6 & K1 giants) and Procyon (an F5 IV-V) plus the Sun. The ultraviolet data for these targets was from: Cen (Ayres et al. 1983), Ori (Ayres et al. 1983, 1988), Procyon (Ayres, 1991), Capella (Ayres, 1984), Gem (Ayres et al. 1984), AU Mic (Quin et al. 1993, Maran et al. 1994). Radii and distances used were those given by the above authors. The above data enables us to produce an EM curve over the temperature range 104 to 2 105K. For the temperature range up to 107K we simply took the EM curves constructed by Schrijver et al. (1995) and Brickhouse et al. (1995) based on EUVE data. As a check on the agreement between the atomic data used here and those of the above authors, we de-archived the EUVE data for Cen. As can be seen from Fig. 1a the agreement between this analysis and that of Schrijver et al. is well within probable errors in the atomic data. Note, we do not include the high temperature (> 107K) tail as derived by Schrijver et al. as it is not yet clear whether this is real or due to errors in the background correction. For the average `quiet' Sun, the derived EM curve is similar to that of Raymond & Doyle (1981b), who used the observational data of Vernazza & Reeves (1978), see Fig. 2. In the construction of these EM curves, an electron pressure of 1015 cm?3 K was used for Cen, Procyon and Capella, 2 1015 cm?3 K for Ori, 4 1015 cm?3 K for AU Mic, 9 1014 cm?3 K for Gem and 5 1014cm?3 K for the Sun. 3. Results and Discussion As can be seen from an inspection of Figs. 1&2 there are dierences in the solar EM curve and that for Cen. It is however likely that errors in the abundances are the cause of the discontinuities in the resulting Cen EM curve compared to the smoother solar EM curve. These probable errors are however not a major problem since we are interested in the resulting radiative power output of the star. To derive the power output we fold in the radiative loss function (i.e. the sum of the emitted radiation from bound-bound, bound-free and free-free transitions at a particular temperature into the emission measure curve, Total radiative power output from the hot outer layers for our six late-type stars plus the Sun Star log P (erg s?1 ) Table 1. Cen AU Mic Capella Procyon Gem Ori Sun 29.15 30.16 32.11 30.50 32.37 30.29 28.93 see for example Doyle et al. 1989). This has the eect of diminishing errors in abundances as the radiative loss function is `almost' the inverse of the EM curve. Throughout, we used the radiative loss function of Cook et al. (1990) which was derived using the same solar abundance as used here. A summary of the derived radiative power output is given in Table 1. However, we did check what changes dierent abundance can make to the total radiative output by deriving the EM curves assuming the photospheric abundances of Meyer (1985). This produced 30% variation in the nal gure, these errors been similar to that of the atomic excitation rates coecients. However, even the above emission measure technique is time consuming and requires a good selection of spectral lines, which can only be provided by a variety of dierent instruments on dierent satellites, not to mention a good knowledge of atomic physics. In looking into this problem for the Sun, Bruner & McWhirter (1988) showed that, for a variety of dierent solar plasma, the total radiated power output, exclusive of hydrogen line radiation, may be estimated even if one knows the intensity of only one spectral line (either a line formed in the transition region or corona). For example, the above authors produced a relationship between the radiative output from the CIV 1548 A line (PCIV ) and the total radiative output (Ptotal ) of the form logPtotal = 2:17 + 1:08 logPCIV (both quantities in erg cm?2 s?1 ). This then takes us back to our original question; would this approximation operate for stars other than the Sun as has been suggested by Doyle (1989)? In Fig. 3 we plot the radiative output based on the emission curves derived from the IUE/HST/EUVE data and that from the C IV 1548 A line for the average surface losses and for the whole star. As is obvious from the gure, excellent agreement exists; thus, surprising as it may seem, the answer may be that one transition region line is sufcient. Furthermore, there is no evidence of a saturation eect, although we should note that this approximation does not include losses due to hydrogen which Houdebine et al. (1995a) have shown to be signicant. Also, Houde- Fig. 3. (a) The radiative output at the stellar surface (in erg cm?2 s?1 ) versus the C IV 1548 A ux for six bright UV/EUV objects (asterisks) plus a selection of dierent solar features including ares, active regions, sunspots, etc. (open circles) given by Bruner & McWhirter (1988) and (b) the total radiative power (in erg s?1 ) versus the radiative output from the CIV 1548 A line for our six stellar sources plus the average `quiet' Sun bine et al. (1995b) have shown that in order to estimate EUVE spectroscopic data for a larger selection of objects. the total radiated output, the UV continuum is important, particularly for the very active stars. Acknowledgments Research at Armagh Observatory is grant-aided by the Dept. of Education for N. Ireland. We also acknowledge the support provided in terms of both and hardware by the STARLINK Project which However, it is clear from Fig. 3 that use of the C issoftware funded the UK PPARC. JGD wishes to thank the IV 1550 multiplet is a valuable tool in estimating the Center forbyEUV for funding which enabled losses from the upper chromosphere, transition region and a short-term visit.Astrophysics I would also like to express my thanks corona and therefore can be used to infer information on to Drs. M. Mathioudakis & J. Drake for their hospitality the magnetic heating requirements of these regions. This during my stay in Berkeley. relationship is valid for the less active stars such as the Sun where the chromospheric lines are a signicant fraction of the radiative output to the active stars such as AU Mic where the coronal region is the dominant source. It is not Ayres, T.R., Linsky, J.L., Simon, T., Jordon, C. & Brown, A., 1983, ApJ 274,784 the intention here to indicate that C IV 1550 A is the only line that can be used as an indicator of the radiative power Ayres, T.R., Simon, T. & Linsky, J.L., 1984, ApJ 279,197 output. This line is formed near 105K which in energetic Ayres, T.R., 1984, ApJ 284,784 terms is relatively unimportant compared to 104K or Ayres, T.R., Jensen, E. & Ergvold, O., 1988, ApJS 66,51 Ayres, T.R., 1991, ApJ 375,704 106K as the emission measure in the region is low. As Rutten et al. (1991) showed in their study of ux-ux re- Brickhouse, N.S., Raymond, J.C. & Smith, B.W., 1995, ApJ (in press) lations involving Si IV 1396 A, C IV 1550 A, C II 1335 A and broad-band X-ray uxes, these are all close to linear, Bruner M.E. & McWhirter R.W.P., 1988, ApJ 326,1002 thus there are several lines which could be used in this Cook L.W., Cheng C.-C., Jacobs V.L. & Antiochos S.K., 1989, ApJ 338,1176 type of analysis. In-fact, Bruner & McWhirter (1988) did consider several other lines. The main reason for using C Doyle J.G. & Keenan F.P., 1992, A&A 264,173 IV 1550 A is that this is the strongest line observable in the Doyle, J.G. 1987, MNRAS 224, 1 short wavelength camera onboard IUE, and thus observa- Doyle, J.G. 1989, A&A 214, 258 tional data exits for a large selection of late-type objects. Doyle J.G., Byrne P.B., & van den Oord G.H.J., 1989, A&A 224,153 Further work is however required on obtaining additional References press) Feldman, U., Mandelbaum, P., Seely, J.F., Doscheck, G.A. & Gursky, H., 1992, ApJS 81,387 Herbst, W. & Miller, J.R. 1989, AJ 97, 891 Houdebine, E.R., Doyle, J.G. & Koseielecki, M., 1995a A&A 294,773 Houdebine, E.R., Mathioudakis, M., Doyle, J.G. & Foing, B.H., 1995b A&A (in press) Jordan, C., Ayres, T.R., Brown, A., Linsky, J.L. & Simon, T. 1987, MNRAS 225,903 Linsky, J.L. 1991, Mechanisms of Chromospheric and Coronal Heating, eds. Ulmschneider, P., Priest, E.R. & Rosner (Springer-Verlag), 166 Maran, S.P., Robinson, R.D., Shore, S.N., Brosius, J.W., Carpenter, K.G., Woodgate, B.E., Linsky, J.E., Brown, A., Byrne, P.B., Kundu, M.R., White, S., Brandt, J.C., Shine, R.A. & Walter, F.M., 1994, ApJ421,800 Mathioudakis, M. & Doyle, J.G. 1992, A&A 262, 523 Meyer, J.P. 1985, ApJS 57,173 Quin, D.A., Doyle, J.G., Butler, C.J., Byrne, P.B. & Swank, J.H., 1993, A&A 272,477 Raymond J.C. & Doyle J.G., 1981a, ApJ 245,1141 Raymond J.C. & Doyle J.G., 1981b, ApJ 247,686 Rutten, R.G.M., Schrijver, C.J., Lemmens, A.F.P. & Zwaan, C., 1991, A&A 252,203 Schrijver, C.J. 1987, A&A 172, 111 Schrijver, C.J., Mewe, R., van den Oord, G.H.J. & Kaastra, J.S., 1995, A&A (in press) Vilhu, O. 1987, Cool Stars, Stellar Systems and the Sun, Lecture Notes in Physics 291, eds. J.L. Linsky & Stencel, R.E. (Springer-Verlag), 110 Vernazza, J.E. & Reeves, E.M., 1978, ApJS 37,485. This article was processed by the author using Springer-Verlag LATEX A&A style le L-AA version 3.